1. Field of the Invention
The present invention relates to a fuel cell power generation system including a fuel cell stack, a fuel processing apparatus, an air supply apparatus, and a power varying apparatus. More particularly, the present invention relates to a fuel cell power generation system which has improved reaction gas supply and exhaust mechanisms and which can control effective power generation surface area corresponding to load ratio.
2. Description of the Prior Art
FIG. 1 is a perspective view showing an example of the structure of the unit cell of a well known fuel cell. FIG. 2 is a plan view schematically showing the conventional fuel cell stack. FIG. 3 is a system flow chart in a simplified view showing the well known construction of a fuel cell power generation system.
As shown in FIG. 1, a unit cell 100 includes a matrix 101 holding an electrolyte, and a pair of a fuel electrode 102 and an air electrode 103 together sandwiching the matrix 101. The pair of the electrodes 102 and 103 are each permeable to gases and provided with an electrode catalyst layer (not shown) on their respective sides contacting the matrix 101. On the surface of the fuel electrode 102 opposite to the matrix are formed a plurality of fuel gas passages 102A in the form of a groove. On the other hand, on the surface of the air electrode 103 opposite to the matrix are formed a plurality of air passages 103A. The fuel as passages 102A and air passages 103A are arranged perpendicular to each other.
A plurality of the unit cell 100 having the aforementioned construction are laminated alternately with gas-impermeable separators 104 to form a fuel cell stack 1 as schematically shown in FIG. 2. As shown in FIG. 2, a pair of inlet and outlet manifolds 2A and 2B for circulating the fuel gas are attached sealingly on side wall surfaces of the stack 1 where both ends of a number of the fuel gas passages 102A open. The fuel gas inlet manifold 2A, the fuel gas outlet manifold 2B and a number of the fuel gas passage 102A together form a fuel chamber 2. Also, a pair of inlet and outlet manifolds 3A and 3B for air circulation are attached sealingly to side wall surfaces of the stack 1 that are perpendicular to the side surfaces to which the manifolds 2A and 2B are attached sealingly. The air inlet manifold 3A, the air outlet manifold 3B and a number of air passages 103A together form an air chamber 3.
In the stack 1 having the aforementioned construction, as shown FIG. 3, the fuel chamber 2 is connected to a fuel reformer 5. Fuel gas such as fossil fuel or hydrocarbon fuel stored in a raw material tank 4 is sent to the reformer 5 through a pump 8A. Hydrogen-rich fuel gas GF produced by steam reforming reaction in the reformer 5 is fed to the fuel chamber 2. On the other hand, air for reaction GA is fed to the air chamber 3 by a blower 8C, and off-air OA is discharged from the air chamber 3. As a result, the stack 1 performs electromotive reaction due to electrochemical reaction between the pair of electrodes 102 and 103, and the electric power generated is supplied to a load 9 after being adjusted for its output current waveform, voltage or the like by a power transformer 6. Off-gas GO discharged from the fuel chamber 2 is sent to a burner of the fuel reformer 5 and is mixed with air sent from the blower 8B to burn. The heat thus generated is utilized as heat source for reaction heat required for the steam reforming reaction. The fuel processing apparatus including the power transformer 6, the air feeder mainly composed of the blower 8C, the fuel tank 4, the pump 8, the fuel reformer 5, and the air blower 8B, is controlled by a controlling apparatus 7. The controlling apparatus 7 also controls the amount of power generated by the stack 1, output electric current from the power transformer 6 and the like depending on the power required by the load 9.
Generally, there are some limitational conditions on the operation of the fuel cell power generating system. One of them is a condition that the fuel cell must not be exposed to a high potential (0.8 V/cell or higher) at a high temperature (about 130.degree. C. or higher). The reason for this is as follows. That is, the catalyst layer of the electrodes of the fuel cell is made of a catalyst composed of platinum or platinum alloy carried on fine particles of carbon such as carbon black, and in the catalyst layer on the electrode particles of platinum or platinum alloy contained therein tend to be dissolved and redeposited. This redeposition results in coarsening of the particles of platinum or its alloy to reduce the surfaces of the platinum, i.e., reaction surface area. Or, the fine particles of carbon carrying thereon platinum or its alloy in the catalyst layer tend to be corroded because of high potential. As a result of this corrosion, platinum is removed from the catalyst layer to decrease the reaction surface area (surface area of platinum) in the catalyst layer. When decrease of reaction surface area occurs at such a high potential as described above, this naturally results in aggravation of the properties of the fuel cell.
In fuel cells, the output voltage V decreases according as output current I increases, which phenomenon is called V-I characteristics. When the output current I reaches a value of, for example, no more than 25% of the rated current (hereafter, expressed as "no more than 25% of load factor"), the voltage of unit cell exceeds 0.8 V, and the unit cell is exposed to a high potential. In the conventional stack where the reaction gas GF and the reaction air GA flow uniformly through a plurality of unit cells over entire electrode surface area and in addition streams flowing perpendicular to each other are formed as shown in FIG. 2, it is unavoidable that each unit cell is exposed to a high potential as high as exceeding 0.8 V/cell under a light load condition at a load factor of no higher than 25%. Therefore, it has been necessary to take a suitable countermeasure for preventing the load factor from decreasing to 25% or lower even under light load conditions. For example, a discharge resistor has been connected to the output side of the stack 1 through a switch in order to direct most part of the output current to the discharge resistor so that the apparent load factor should not decrease to 25% or lower. However, this type of countermeasure is disadvantageous in that the consumption of reformed fuel increases according as power generated is wasted, thus decreasing power generation efficiency. Further, in this countermeasure, accessories to the discharge resistor, switch and the like, or controllers therefor are necessary because of waste of electric power, which results in increase in the size of the power generation system.
In particular, in recent fuel cells, improvement of the performance of the battery has been made in order to cope with technical developments such as the use of a higher pressure reaction gas, and as a result the output voltage of fuel cells has increased to some extent, and unit cells could readily be exposed to a high potential exceeding 0.8 V/cell even at a load factor of 50% or higher, not to speak of 25% or lower.